Friday, November 20, 2015

Back to Basics: What’s the deal with Magnets?

I consider myself a fairly sharp guy. I've made a living off of being a scientist for over 20 years now, and I have at least a passing knowledge of most scientific fields outside my area. But I feel like I should be able to do something other than babble incoherently when asked about magnets. They baffle me – there, I said it. So what do I do about it? Write a Hackaday post, naturally – chances are I'm not the only one with cryptomagnetonescience, even if I just made that term up. Maybe if we walk through the basics together, it'll do us both some good understanding this fundamental and mysterious force of nature.

For this article, I'm mostly interested in permanent magnets. There's something primal and universal about playing with permanent magnets, and feeling that invisible force field holding apart two magnets with the same poles facing each other is compelling in a way that few other science experiences are – except for maybe getting the same two magnets to stick to each other through the web between your thumb and forefinger. But what makes these two inert discs or balls or horseshoes act like that? Where does the seemingly inexhaustible energy to repel and attract come from? How do permanent magnets work?

Hitting the Books

My first stop on this journey was to go to the bookshelf and find an old physics book. I found the one I used in my undergrad days, Serway's Physics for Scientists and Engineers. As fascinating as the book was, it didn't help. A quarter century has passed since I cracked that very dusty book, and all I learned from it is that you can't go back to your college days.

So next I looked around the web for some answers. At the end of the day, no matter whom you talk to about magnets and no matter how many words are stuffed into the explanation, the honest answer to how magnets work is, "We just don't know." We know a lot of things about magnetism – that it's a property of space, for instance, and that the force field of magnets is composed of photons. But knowing that you can only explain permanent magnets in terms of quantum mechanics isn't terribly helpful in my efforts to stop the hand waving. Time to turn to YouTube.

I dialed up a MinutePhysics video on magnets, and I found it really helpful. My main takeaway from the segment was that permanent magnets are best understood as really small electromagnets. It seems ironic that to explain a magnet with only one part we need to think in terms of a magnet made from wire coiled around a core and attached to a source of electricity, but there it is.

A basic fact of nature is that charged particles have intrinsic magnetic moments, which basically means they're really small magnets. Electrons and protons are charged particles, so all matter is made of tiny magnets. It turns out that the protons are really weak, so the nucleus isn't really invited to the magnet party being run by the electrons. This is convenient because we only have to look at the electrons, but also infuriating because we have to deal with the whole concept of electron shells. I'm not going to relive that hellish little section of Chemistry 101 except to say that in atoms with a filled shell, the magnetic field generated by any moving electron is going to be cancelled out by another electron in the same shell moving in the opposite direction. Also, filled shells have electrons in pairs, but their intrinsic moments are opposite of each other, and they also cancel each other out. So, no net magnetic field from atoms with filled shells.

But, in an atom with a half-filled shell, the electrons are unpaired, and their intrinsic moments are the same polarity. All those tiny magnets add up, and you've got yourself a magnet. And, because of the half-filled shells thing, it's easy to spot which elements are likely to be magnetic on your periodic table. Half-filled shells occur near the middle of the f-block (the actinides and lanthanides) and the d-block (the metals). Notice that the top row of the d-block has all the "classic" magnetic elements – cobalt, manganese, chromium, nickel and the king of them all, iron. Ever heard of Alnico? It's iron alloyed with aluminum, nickel and cobalt, and it's used to make permanent magnets because it has high coercivity, which means that once it's magnetized, it stays that way.

How to Make a Magnet

So how exactly are permanent magnets made? There are a lot of methods, most of which are basically some sort of metal manufacturing process, like casting, machining, or sintering. Most magnets undergo multiple operations, especially the super-strong rare-earth magnets, which also require extra protective plating with nickel to prevent corrosion. As an aside, the nickel plating stands up remarkably well after a two-day tour of the human digestive tract. Source: I'm a dad.

Once the metalwork is complete, the magnet still needs to be magnetized. There are a number of ways to accomplish this, but it basically seems to involve dumping a ridiculous number of electrons into a coil near the baby magnets and inducing a huge magnetic field that aligns the magnetic domains permanently. Notice the ammeter in this "How It's Made" video; at the 3:53 mark, it's reading 6,000 amps!

There's another way to make a magnet that doesn't involve banks of supercapacitors. In fact, blacksmiths have known for centuries that beating on hot metal can make it magnetic. When a ferromagnetic metal is heated past its Curie temperature, it loses its magnetic properties – see this video for a neat demonstration. Once past that temperature – about 1400°F for iron, or a light cherry red – the magnetic domains can be re-aligned by pounding on the metal while it's oriented within a magnetic field, like the Earth's. Better results can be achieved with a stronger magnetic field – for all it does to protect us from cosmic radiation, the Earth's magnetic field is really kind of weak.

So in the final analysis, a permanent magnet is really just a device that captures a little bit of the magnetic field of another, stronger magnet. In other words, if you want to magnetize something, you need to move electrons. There's no separating electricity and magnetism – they're two sides of the same coin.

I'm glad I poked my head down the quantum rabbit hole that is magnetism. I'm nowhere near a complete understanding of how permanent magnets work, but I'm a little closer to it. Maybe I can stop the hand waving now and sound a little more authoritative on the subject.

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